1. Field of the Invention
The present invention relates to a detachable developer used for a laser printer or a photocopier, and more particularly, to a detachable developer which can maintain a uniform development nip even when a photosensitive unit and a development unit are independently installed.
2. Description of the Related Art
A developer used for a laser printer, a photocopier, or a multifunctional printer is provided in the form of a cartridge to print a processed image on paper. Conventionally, a photosensitive unit and a development unit are installed together in a single cartridge. A latent electrostatic image is formed on the photosensitive unit by being exposed to a laser. The development unit forms a toner image corresponding to the latent image by supplying toner to the photosensitive unit. However, since the life span of the photosensitive unit is much longer than that of the development unit, if the life span of the development unit ends, the photosensitive unit is replaced together with the development unit, even though the photosensitive unit does not need replacement.
To solve the above problem, as shown in FIG. 1, a structure to separately drive a photosensitive unit 100 and a development unit 200 has been suggested. According to this structure, the photosensitive unit 100 and the development unit 200 are independently installed on a main body of a printer or a photocopier and a driving force from the main body is independently transferred. Referring to FIG. 1, the structure of each of the photosensitive unit 100 and the development unit 200 is described.
The photosensitive unit 100 includes a photosensitive drum 120 and a charge roller 140 which are assembled at their respective positions in a photosensitive unit housing 110. The photosensitive drum 120 is partially exposed from the photosensitive unit housing 110. A photosensitive unit coupling member 130 to transfer the driving force from the main body to the photosensitive unit 100 is coupled to an end portion of a rotation axis of the photosensitive drum 120. The photosensitive unit housing 110 is fixed in a main body frame (not shown) such as a printer.
The development unit 200 includes a developing roller 210 to provide the photosensitive drum 120 with toner, a toner supply roller 220 to supply the toner to the developing roller 210, a development unit coupling member 250 to transfer a driving force from the main body to the development unit 200, a gear train 240 having a plurality of gears to transfer the driving force of the development unit coupling member 250 to the developing roller 210 and the toner supply roller 220, and a development unit housing 230 to which the above parts are fixed. The developing roller 210 is partially exposed outside the development unit housing 230 to contact the photosensitive drum 120. When the development unit 200 is installed at the main body, the development unit coupling member 250 is connected to a driving apparatus connecting unit (not shown) of the main body.
Typically, in the conventional detachable type developer having the above structure, the photosensitive unit 100 is fixed to the main body. To form an image, the developing roller 210 and the photosensitive drum 120 contact each other while maintaining a particular contact width, that is, nip, and a nip depth. The development unit 200 is pressed against the photosensitive unit 100 by a force of a spring (not shown) when the development unit 200 is installed at the main body. The development unit coupling member 250 is connected to a driving apparatus connection unit (not shown). During the transfer of the driving force, the entire development unit 200 can swing around an axis. That is, one end portion of the development unit coupling member 250 is connected to the driving apparatus connection unit (not shown) fixed in the main body and the other end portion thereof is fixed to the main body frame. Thus, the development unit coupling member 250 becomes a rotation axis of the swing motion and the overall development unit 200 swings around the development unit coupling member 250 so that the development nip is maintained.
In general, in an image forming apparatus using a non-magnetic contact type developing method in which the developing roller 210 and the photosensitive drum 120 are engaged and rotate in contact with each other, the most important requirement for image quality is to maintain a uniform development nip between the developing roller 210 and the photosensitive drum 120. Typically, the nip depth is between 0.05-0.15 mm. If the nip depth is outside the above range, image dirt and toner stress becomes severe due to an excessive nip. In contrast, when the nip is not formed, an image cannot be formed.
Both end portions of the development unit coupling member 250 which serve as the rotation axis of the development unit 200 performing a swing motion are fixed to the driving apparatus connection unit and the main body frame. Accordingly, once the development unit 200 is installed at the main body, the position thereof is permanently fixed. Therefore, an axis arrangement can deviate due to an error generated during assembling and manufacturing processes and also due to a torque generated during transfer of power. In particular, since the length in the axis direction increases in the case of using paper having a size greater than A4, it is more difficult to align the axis stably. FIG. 2 shows a case in which the positions of the axis are incorrect at both end portions of the development unit coupling member 250. Referring to FIG. 2, the developing roller 210 and the photosensitive drum 120 contact each other by being deviated from each other. When the developing roller 210 and the photosensitive drum 120 are deviated from each other, the development nip becomes unstable so that a particular part of the paper is correctly printed while a print state in the other part thereof greatly deteriorates.